The present invention relates to deepwater offshore platforms. More particularly, it relates to single caisson, tethered structures.
Small, minimum-capability platforms have several advantages over large, full-capability platforms in the development of hydrocarbon reserves in deep water. A much lower capital cost is one of the significant advantages. However, minimizing platform capability by eliminating a resident drilling rig and other useful equipment from the design also significantly limits the ability of the platform to adapt to new reservoir and/or economic information suggesting changes in the development scenario. The Tension Leg Well Jacket (TLWJ) concept was developed to address this limitation. In the TLWJ concept, a small TLP (the TLWJ, mini-spar or other minimal structure) supports the wells for surface accessible completions, but drilling and other major well operations are performed by a semisubmersible drilling rig which docks to or is otherwise restrained adjacent the TLWJ. This method of conducting well operations is more fully discussed in U.S. Pat. No. 5,199,821, issued Apr. 6, 1993 to D. A. Huete et al for a Method for Conducting Offshore Well Operations and U.S. patent application Ser. No. 024,584, filed by A. G. C. Ekvall et al on Mar. 1, 1993, now U.S. Pat. No. 5,439,324, for a Bumper Docking Between Offshore Drilling Vessels and Compliant Platforms, the disclosures of which are hereby incorporated by reference and made a part hereof.
It is understood that the smaller the floating platform, i.e., the smaller the total hull displacement, the cheaper it is. Although the size of the floating platform is mostly determined by the topsides payload demand and the number of production wells to be supported, there is a point below which the traditional rectangular hull having four comer columns connected at the keel with four horizontal pontoons is no longer an optimal configuration. Revised configurations that support the same amount deck load with shorter deck spans have cost advantages for such minimal configurations. Single column type designs have been developed to serve this need, including monopod and mini-spars, which provide the logically smallest floating platform that is moored with one or more vertical tension members.
A difficulty with the monopod and mini-spar designs are that they tend to roll and pitch (rotate about two horizontal axes), although restrained in heave (vertical motion) by the tendons. The pitch and roll responses of a monopod are troublesome because of fatigue problems in the tendons due to bending, and because of potential interference with well risers which may be arranged outside the column.
Another benefit of the decreasing the size of the structure is that lower loads on the tendons expands the scope of suitable materials for forming the tendons. Thus, full capability platforms have used thick walled tubular goods to form the tendons. These are expensive to produce and relatively difficult to deploy.
By contrast, wire rope tendons would be desirable in this fighter service for their economy and ease of installation. However, there is another contrast between tubular goods and wire rope in tendon applications. Tubular goods have greater reliability, in large part because of inspectability in manufacture and in service. The cylindrical walls of such tubular goods may be inspected inside and out. In contrast, it is more difficult to determine whether a wire rope has suffered damage because the majority of the load-carrying portion is hidden from view.